1. Field of the Invention
The present invention relates to a device for telescopic fork legs, preferably on a motorcycle or bicycle, where the telescopic fork leg comprises outer and inner legs and a damping system with a piston and piston rod arrangement that is arranged within these.
2. Description of the Related Art
A front fork for a motorcycle or a bicycle can be subjected to wheel speeds in the whole range of 0-10 m/s and stroke lengths of up to 300 mm. In order to be able to absorb such high speeds and such large strokes, great demands are made of the front fork. It must be able to absorb forces and be strong, while at the same time it must be able to handle a large flow of oil. It is also desirable to have good control in the whole range of speeds and for the damping to be adjustable. A compact and light system that can be adapted to fit several different front fork dimensions also is desired. Reference is made, for example, to patent U.S. Pat. No. 6,260,832, that shows a front fork of the type described above. U.S. Pat. No. 6,260,832 does not, however, have the desirable build-up of pressure that is described below.
Current systems can be represented by a damper of the De Carbon type, see for example FR1055443A, and have a serial damping force construction that is based on a principle of pressurizing two locations in series in order to avoid cavitation or the admixture of air into the damping medium. This system has limitations in that the pressures in the two pressurizing locations must more or less harmonize with each other, as the drop in pressure (ΔP1=Plow−Pmid, ΔP2=Pmid−Pgas) across the two pressurizing pistons should be greater than zero in order not to create cavitation. See FIG. 1. Because the drop in pressure across the piston is dependent upon the flow resistance through the piston in combination with the force that acts on the piston, the flow resistance, controlled for example by a shims stack, can only be adjusted within a certain limited range, which thus also results in a limited area of use for the damper. It is then also necessary to dimension pistons, piston rod and damping tubes so that the force absorption agrees with the pressures that have been built up, in order to obtain the required damping. With serial damping, the oil is forced through both of the valves in series, which results in high flow speeds. With high flow speeds and high piston speeds, the design of the pistons is limited in order not to obtain an unwanted uncontrolled build-up of pressure due solely to the flow resistance.
A system with parallel damping solves the abovementioned problem. Examples of such dampers can be found in the patent documents EP1505315A2 and EP0322608A2. The parallelism in the damping arises through the damping medium being pressurized by two pressurizing pistons that are arranged parallel to each other in the damping chamber and in a space arranged outside the damping chamber. The pressurized outer space is interconnected with both the compression chamber and the return chamber. With parallel damping, the pressure on the low-pressure side of the damping piston is always as large as possible, irrespective of whether the front fork is subjected to a compression or a return stroke. The definition of the low-pressure side of the damping piston is the side of the piston where the volume of the chamber increases. Due to the fact that the pressure is never allowed to become zero on that side, cavitation is prevented. This parallel arrangement also means that the damper can be pressurized and the pressure, that is the damping, can be adjusted without having to take into account the drop in pressure across the piston(s). The increase in pressure, as well as the increase in force, now takes place without cavitation, irrespective of the setting.